Studies on the possible cause of Beech leaf disease in North America
Beach leaf disease (EPPO Alert List) was first observed in 2012 in the Lake county in Ohio (US) on Fagus grandifolia (American beech) and has since spread across Northern Ohio, Western and Northern Pennsylvania, New York, Ontario (Canada) and Southwestern Connecticut (EPPO RS 2018/178,2019/083, 2019/083,2020/083). This disease has also been observed on Fagus sylvatica (European beech) in Ohio. Beech leaf disease is characterized by dark interveinal banding of leaves appearing soon after spring flush, and in advanced stages results in canopy thinning, followed in some cases by tree mortality. Although the cause of this disease remains uncertain, nematodes could be extracted from symptomatic leaves of F. grandifolia and F. sylvatica from North America and were initially found to be most similar to Litylenchus crenatae. This nematode species was first described in 2019 in Japan in association with leaf galls symptoms on Fagus crenata (Japanese beech).
More studies have been conducted in North America to try to elucidate the cause of beech leaf disease. Nematode populations isolated from symptomatic beech leaves (F. grandifolia and F. sylvatica) collected in Ohio (initially identified as L. crenatae), Pennsylvania and Ontario were further studied. Results showed that North American nematode populations differed in morphology, host range, and ribosomal DNA marker from those in Japan. Therefore, it was proposed to consider the North American nematode associated with beech leaf disease as a new subspecies of L. crenatae, and to call it: Litylenchus crenatae mccannii. Inoculation of beech (F. grandifolia) seedlings with freshly isolated L. crenatae mccannii nematodes resulted in beech leaf disease symptoms, thus confirming that the nematode plays a role in the disease observed in North America (Carta et al., 2020).
Because the presence of L. crenatae mccannii has been observed on both symptomatic and asymptomatic bud and leaf tissue, the question as to whether nematode is the sole cause of the disease or is only a vector of unknown pathogens, remains largely unanswered. Studies have been conducted on the fungal and bacterial communities that can be found on leaves and buds of F. grandifolia (asymptomatic and symptomatic). The lack of differences between the fungal communities occurring on symptomatic and asymptomatic tissues suggested that fungi do not play a role in the disease symptomatology. However, it was observed that bacterial communities were significantly different between symptomatic and asymptomatic leaves, in particular for the genera Wolbachia (many species are known as insect symbionts) and Mucilaginibacter (known as saprophytic bacteria able to degrade pectin). The differences observed in Wolbachia populations might suggest the possible involvement of an insect vector that could spread the nematode between leaves and trees. Concerning differences observed in Mucilaginibacter, this might suggest the possible involvement of an endosymbiont that could facilitate nematode feeding and establishment on leaves. Following these preliminary results, it is acknowledged that further studies should be carried out to determine what is the role of these bacteria in the colonization of leaves by L. crenatae mccanni and the progression of the disease in beech trees (Burke et al., 2020).
Sources
Burke DJ, Hoke AJ, Koch J (2020) The emergence of beech leaf disease in Ohio: probing the plant microbiome in the search of the cause. Forest Pathology, e12580. https://doi.org/10.1111/efp.12579
Carta LK, Handoo ZA, Li S, Kantor M, Bauchan G, McCann D, Gabriel CK, Yu Q, Reed S, Koch J, Martin D, Burke DJ (2020) Beech leaf disease symptoms caused by newly recognized nematode subspecies Litylenchus crenatae mccannii (Anguinata) described from Fagus grandifolia in North America. Forest Pathology, e12580. https://doi.org/10.1111/efp.12580